Compact high-power broadband radiofrequency load termination

ABSTRACT

A load termination for absorbing a broad range of radiofrequency energy and comprised of a large number of aligned chambers formed by closely spaced plates that are coated with a lossy material and are spaced apart a distance that is substantially less than the spacing required for the chambers to resonate at the highest frequency of the range.

United States Patent Price et al.

[ 5] Feb. 8, 1972 [54] COMPACT HIGH-POWER BROADBAND RADIOFREQUENCY LOAD TERMINATION [72] Inventors: Vernon G. Price, I408 Altos, Califi;

Richard W. Grow, Salt Lake City, Utah [73] Assignee: The United States of America m represented by the United States Atomic Energy Commission 22 Filed: Sept. 15,1970

l2l] Appl.No.: 72,295

[52] U.S.CI. ..333/22R,333l81,333l98 [5]] Int. Cl. H01]: 1/26 [58] Field oI'Search ..333/22, 81,98

[56] References Cited UNITED STATES PATENTS 3,309,634 3/1967 Wheeler et al ..333/22 X 2,939,993 6/1960 Zublin et al 3,03 6,280 5/1962 Woodcock ..333/22 3,050,606 8/1962 Tibbs ....333/8I X 3,264,515 8/1966 I-Iaimson..... ....333/22 X 3,437,866 4/1969 Hentschel ..333/8l X FOREIGN PATENTS OR APPLICATIONS 732,683 6/ I955 Great Britain ..333/22 OTHER IUBLKA'IIONS The Bell System Technical Journal Vol. 35 Sept. I950 TKI B435: pages ll26-l 128 Primary Examiner-Herman Karl Saalbach Assistant ExaminerMarvin Nussbaum Attorney-Roland A. Anderson ABSTRACT A load termination for absorbing a broad range of radiofrequency energy and comprised of a large number of aligned chambers formed by closely spaced plates that are coated with a lossy material and are spaced apart a distance that is substantially less than the spacing required for the chambers to resonate at the highest frequency of the range.

10 Claims, 2 Drawing Figures PATENTEDFEB 8!.972 3,541,455

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INVENTORS VERNON 6. PRICE BY RICHARD W. GROW Maw ATTORNEY COMPACT HIGH-POWER BROADBAND RADIOFREQIUENCY LOAD TERMINATION BACKGROUND OF THE INVENTION The present invention relates to terminations for radiofrequency waves, and more particularly, it relates to a multiplicity of small, nonresonant, closely spaced and aligned cavities that are coated with a lossy material.

Various types of load terminations for absorbing radiofrequency energy at the end of a waveguide are well known. Among these types there are long tapered terminations, waterload terminations, resonant cavity tenninations, a tennination of a lossy material embedded in a binder in a tapered shape, and a termination that comprises a corrugated metallic surface having a thin resistance sheet overlying the ends of the corrugations. However, each of these types of load terminations has disadvantages: a long tapered termination requires a large space and is subject to multipactoring; a waterload termination requires a radiofrequency window between the termination and the waveguide, which window is susceptible to breakage with consequent leakage of water into the waveguide system; resonant cavity terminations are narrow band; a termination comprised of lossy material in a' binder is subject to excessive outgassing; and a termination comprised of a corrugated metallic surface with a resistance sheet overlying the ends of the corrugation is subject to arcing between the corrugations through the resistance sheet thereby destroying the sheet and eliminating its ability to absorb radiofrequency energy.

SUMMARY OF THE INVENTION In brief the present invention is a load termination for absorbing a broad range of radiofrequency energy. The termination is comprised of a large number of aligned chambers that are formed by closely spaced thin plates that are coated with a lossy i.e., resistive material and spaced apart a distance that is substantially less than the spacing required for the chambers to resonate at the highest frequency of the range of radiofrequencies to be absorbed. The close spacing of the plates permits a very large number of chambers to be used in a compact space; and since each chamber absorbs a significant amount of energy the large number of chambers gives the termination the capacity to absorb very large amounts of energy at high-power levels. Furthermore, by spacing the plates at a distance that is less than the spacing required for resonance at the highest frequency of the range, the extremely high-power levels that would otherwise build up in the chambers under resonant conditions do not occur. Thus, the energy in each chamber is maintained at a level that will not cause arcing and consequent deterioration of the chamber. In addition, since the termination does not depend on resonant chambers for absorption of large amounts of energy in each chamber but rather on a large number of nonresonant chambers each of which absorbs small but significant amounts of energy, the termination is effective for absorption of large amounts of radiofrequency energy over a wide range of frequencies, thereby making the termination useful over a broad band of frequencies without adjustment of the termination or replacement of the termination with another for each new frequency. Another advantage of the termination of the present invention is that multipactoring is eliminated since the dimensions of the termination are constant from chamber to chamber. Also, the termination is susceptible of being made of materials having a high heat-conducting capacity so that the large amounts of energy absorbed by the termination may be efficiently conducted away from the termination and dissipated. The termination of the present invention is also susceptible of being made of materials that are not gassy, i.e., they are nonporous and can withstand bakeout temperatures to 600 C., thereby permitting the termination to be connected directly to a waveguide.

It is an object of the invention to provide a compact highpower broadband radiofrequency load termination.

Another object of the invention is to provide a radiofrequency load termination that is durable at high-power levels, that efficiently transfers heat, that is free of multipactoring over a broad band of frequencies, and that is free of out-gassing to permit direct connection to a waveguide system that is under a high vacuum.

Other objects and advantageous features of the invention will be apparent in the description of the specific embodiment thereof, given by way of example only, to enable one skilled in the art to readily practice the invention, and described hereinafter with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION or AN EMBODIMENT Referring to the drawing there is shown in FIGS. 1 and 2 a rectangular waveguide 10 having a load termination 12 connected to the end thereof. The load termination 12 is comprised of a large number of thin rectangular plates 14 each having a small rectangular hole 15 centrally formed in the plate. Each of the holes has dimensions equal to the inside cross-sectional dimensions of the waveguide 10, and each hole is aligned with the waveguide. The plates 14 are each spaced apart by means of a spacer 16 which conveniently may be made from a thin rectangular plate having a length and width equal to the plates 14 and provided with a large central space 17. A solid plate 18 having the same outside dimensions as the plates 14 is connected to the end of theload termination 12, while a plate 20 similar in shape to the plates 14 is connected at the input end of the load termination 12 for connection to the output end of the waveguide 10.

The plates 14, 18 and 20 are stacked together with a spacer 16 between each plate. The plates and spacing members are then brazed together to form an integral vacuumtight unit which may be cooled for example by immersion in circulating cooling water, cooling tubes brazed to the surface of the unit, or cooling passages drilled in the solid periphery of the unit.

The particular arrangement of the load termination 12 provides a-large number of small chambers 22 which are defined by the plates l4, l8 and 20 and the spacers 16. The surfaces of the plates that form the chambers are coated such as by flame spraying with a lossy material such as an alloy having a composition of 20 to 31 percent nickel, 12 to 19 percent cobalt, and 0.2 to 0.5 percent manganese, and the balance iron.

In operation, radiofrequency energy propagates in a first direction from the waveguide 10 to the load termination 12 through the aligned holes 15 towards the plate 18. During its propagation through the load termination 12 part of the energy enters each of the chambers 22 transversely to the first direction. The amount of power P, that enters each of the chambers is R times the power P that is travelling through the holes 15 adjacent the chambers, where R equals d divided by w, d being the distance between the plates 14 and w being the width of the holes 15. The energy entering the chambers 22 propagates toward the outer end of the chambers and in so doing is dissipated in the lossy material with which the walls of the chambers are coated.

By constructing the load termination 12 to have a large number of chambers 22, each with a substantial depth, the radiofrequency energy propagated from the waveguide 10 may be completely dissipated in the load termination 12 even though only small amounts of energy, typically 1 percent of the energy passing through the holes 15 adjacent the chambers, is dissipated in each of the chambers 22. The large number of plates also present a very large surface area for absorption of heat dissipated in the lossy material; and when made of a material having a high thermal conductivity such as lOl022 069l copper, they also constitute a large mass of heat conducting material for efficiently and rapidly conducting heat away from the load termination 12.

In a model exemplifying the invention, a waveguide 12 was constructed in which the plates 14, 18 and 20 and the spacers 16 all were of a nominal width of 4 inches, a length of 12 inches and a thickness of 0.067 inch. Since the spacers 16 in this model were 0.067 inch thick, the distance d between the plates 14 also was 0.067 inch. The windows 15 all had a nominal width w of 1.34 inches and a length of 2.84 inches. All of the spaces 17 were 2.84 inches by 10.38 inches. Each of the chambers 22 had a width of 2.84 inches and a depth of 4.52 inches wherein the depth was made to correspond to three-fourths of the guide wavelength of 15.30 cm. at 'a frequency of 2,856 MHz.

The plates l4, l8 and 20 and the spacers 16 were all made of copper. A lossy material wasflame sprayed on the plates l4, l8 and 20 and was an alloy of nickel, cobalt, manganese and iron having a resistivity of approximately 145 relative to copper or 2.5xlohm-meters. The termination comprised 182 spacers 16, 181 plates 14, one end plate 18 and one end plate 20 resulting in 182 pairs of chambers 22. The termination was connected to the end of a length of standard waveguide having a WR-284 designation by the Electronics Industries Association and having nominal outside dimensions of 3 inches by 1% inches and inside dimensions of 2.84 inches by 1.34 inches. Microwave energy in the range of 10 megawatts peak and 10 kilowatts average power at a frequency of 2,856 MHz was propagated from the waveguide to the termination. The termination provided a power loss of 0.16 db. per pair of chambers 22 for a total power loss of 29 db. The termination was cooled by means of copper cooling tubes brazed to the outer edges of the plates and spacers.

While an embodiment of the invention has been shown and described, further embodiments or combinations of those described herein will be apparent to those skilled in the art without departing from the spirit of the invention.

What is claimed is:

l. A radiofrequency load termination for absorbing a range of radiofrequency energy propagated in a first direction, compnsrng:

a plurality of members having a high heat-conducting capacity, each of said members being coated with a material that is resistive to microwaves, said members being arranged to guide radiofrequency energy in said first direction past at least one edge of each member in a direction that is transverse to the members;

spacing means for spacing successive members apart, said spacing means and said members forming a plurality of chambers, each of said chambers having a width d equal to the thickness of said spacing means, said width being of the order of one-ninetieth of the width required for a chamber to resonate at the highest frequency of said range.

2. The radiofrequency load termination of claim 1 wherein said spacing means and said members and their resistive surfaces are of materials that can withstand a bakeout temperature of 600 C.

3. The radiofrequency load termination of claim I wherein said spacing means and said members and their resistive surfaces are all metal.

4.'The radiofrequency load termination of claim 1 wherein said members have a surface that is an alloy having a composi tion of 20 to 31 percent nickel, 12 to 19 percent cobalt, and 0.2 to 0.5 percent manganese, and the balance iron.

'5. The radiofrequency load termination of claim 4 wherein all of said members are copper and their surface is coated with said alloy.

6. The radiofrequency load tennination of claim 1 wherein said members are equal in size and shape and each has an aperture formed therein and further including a length of waveguide with said temrination connected to said waveguide with said apertures in alignment therewith.

7. The radiofrequency load termination of claim 6 wherein said members and their apertures are rectangularly shaped.

8. The radiofrequency load temrination of claim 1 wherein said members are flat and have a thickness on the order of the width of said chambers.

.9. The radiofrequency load termination of claim 1 wherein the depth of the chambers formed by said members and said spacing means is three-fourths of the guide wavelength of the radiofrequency energy being propagated past the edges of said members.

10. The radiofrequency load temiination of claim 1 wherein the guide wavelength of said radiofrequency energy is 15.30 cm. and the width of said chambers is 0.067 inch. 

1. A radiofrequency load termination for absorbing a range of radiofrequency energy propagated in a first direction, comprising: a plurality of members having a high heat-conducting capacity, each of said members being coated with a material that is resistive to microwaves, said members being arranged to guide radiofrequency energy in said first direction past at least oNe edge of each member in a direction that is transverse to the members; spacing means for spacing successive members apart, said spacing means and said members forming a plurality of chambers, each of said chambers having a width d equal to the thickness of said spacing means, said width being of the order of one-ninetieth of the width required for a chamber to resonate at the highest frequency of said range.
 2. The radiofrequency load termination of claim 1 wherein said spacing means and said members and their resistive surfaces are of materials that can withstand a bakeout temperature of 600* C.
 3. The radiofrequency load termination of claim 1 wherein said spacing means and said members and their resistive surfaces are all metal.
 4. The radiofrequency load termination of claim 1 wherein said members have a surface that is an alloy having a composition of 20 to 31 percent nickel, 12 to 19 percent cobalt, and 0.2 to 0.5 percent manganese, and the balance iron.
 5. The radiofrequency load termination of claim 4 wherein all of said members are copper and their surface is coated with said alloy.
 6. The radiofrequency load termination of claim 1 wherein said members are equal in size and shape and each has an aperture formed therein and further including a length of waveguide with said termination connected to said waveguide with said apertures in alignment therewith.
 7. The radiofrequency load termination of claim 6 wherein said members and their apertures are rectangularly shaped.
 8. The radiofrequency load termination of claim 1 wherein said members are flat and have a thickness on the order of the width of said chambers.
 9. The radiofrequency load termination of claim 1 wherein the depth of the chambers formed by said members and said spacing means is three-fourths of the guide wavelength of the radiofrequency energy being propagated past the edges of said members.
 10. The radiofrequency load termination of claim 1 wherein the guide wavelength of said radiofrequency energy is 15.30 cm. and the width of said chambers is 0.067 inch. 